1. Field of the Invention
The present invention relates to dielectric ceramic compositions which are suitable for the use in high-frequency regions, such as the microwave region and millimeter-wave region. In particular, the present invention relates to a dielectric ceramic composition which is suitable for miniaturization of products by laminating metallic electrode layers and sintering the laminate. The present invention also relates to a monolithic ceramic substrate, a ceramic electronic component and a monolithic ceramic component using the dielectric ceramic composition.
2. Description of the Related Art
In recent years, high-frequency dielectric ceramics have been widely used in, for example, dielectric resonators and dielectric substrates for monolithic integrated circuits (MICs). Major requirements for achieving miniaturization of the high-frequency dielectric ceramics are large dielectric constants, large Q values and small dependencies of dielectric constants on temperature.
Japanese Unexamined Patent Application Publication No. 6-333426 discloses a dielectric ceramic composition having a high specific dielectric constant (xcex5r) and a high Q value in which 0.5 percent by weight or less of MnO2 and 1.2 percent by weight or less of Ta2O5 are added to a main component represented by BaO-x {(1-y)TiO2.yZrO2}. This dielectric ceramic composition is obtained by sintering at a high temperature of at least 1,300xc2x0 C., and has a specific dielectric constant (xcex5r) of at least 38 and a Q value of at least 8,000.
It is necessary to use inexpensive metals having low resistance, such as Ag and Cu, as electrodes for use in high-frequency regions, such as dielectric resonators. Thus, the metal and ceramic must be sintered at a temperature which is lower than the melting point of the metal. The melting points of these metals, however, are in a range of 960xc2x0 C. to 1063xc2x0 C. and are significantly lower than the above-described sintering temperature, that is, at least 1,300xc2x0 C., for the dielectric ceramic composition. Accordingly, the above dielectric ceramic composition suitable for high-frequency regions precludes use of these metals as internal electrode materials.
Japanese Unexamined Patent Application Publication No. 8-45344 discloses a dielectric ceramic composition in which at least one auxiliary component of 0.1 to 5 percent by weight of GeO2 and 0.5 to 5 percent by weight of CuO is added to a major component represented by xc3x97BaO-yTiO2 wherein x is in a range of 0.18 to 0.20, y is in a range of 0.80 to 0.82 and x+y=1. The addition of CuO and GeO2 facilitates sintering at a lower temperature, that is, 1000xc2x0 C.
The GeO2 used in the above composition is, however, relatively expensive and exhibits poor water resistance. Since silver has a high electrical conductivity and is relatively inexpensive, it is most suitable as an electrode material which can be sintered in air. Silver, however, has a melting point of 962xc2x0 C. and thus cannot be simultaneously sintered together with this dielectric ceramic composition.
Japanese Unexamined Patent Application Publication No. 5-325641 discloses a B-TiO2-Zro2 dielectric ceramic composition which can be sintered at a low temperature of 900xc2x0 C. by adding B2O3. B2O3, however, is hygroscopic. Even when a glass component containing excess B2O3 is used instead of the addition of the B2O3, the composition is chemically unstable due to elution of the B2O3 from the glass component. Moreover, the use of the glass component increases cost. Since this composition exhibits relatively low sinterability, the raw materials must be pulverized to an average grain size of 0.6 xcexcm or less before sintering. This is also a factor increasing the cost.
Japanese Unexamined Patent Application Publication No. 10-167817 discloses a dielectric ceramic composition containing 100 parts by weight of BaO-x(Ti1-aZra)O2 as a major component wherein 3.5xe2x89xa6xc3x97xe2x89xa64.5 and 0xe2x89xa6axe2x89xa60.20, 4 to 30 parts by weight, on a ZnO basis, of a zinc compound, 1 to 20 parts by weight, on a B2O3 basis, of a boron compound, 1 to 10 parts by weight, on an alkali metal carbonate basis, of an alkali metal compound, and 0.01 to 7 parts by weight, on a CuO basis, of a copper compound. This dielectric ceramic composition can be sintered at 900xc2x0 C. by the effects of these auxiliary components. This dielectric ceramic composition also contains the boron compound. Since B2O3 in the composition is chemically unstable, the composition exhibits deterioration of insulation resistance in high-temperature, high-humidity environments.
It is an object of the present invention to provide a dielectric ceramic composition which can be sintered together with low-resistance electrode materials, such as silver and copper, at a low temperature of not more than 1,000xc2x0 C., which exhibits a high dielectric constant, a high Q value, a small rate of change in dielectric property with temperature, superior high-frequency characteristics and superior reliability in high-temperature, high-humidity environments.
It is another object of the present invention to provide a monolithic ceramic substrate, a ceramic electronic component and a monolithic ceramic electronic component which use the above dielectric ceramic composition and exhibit superior high-frequency characteristics and superior reliability in high-temperature, high-humidity environments.
According to an aspect of the present invention, a dielectric ceramic composition comprises 100 parts by weight of major component represented by the formula BaO-x {(1-y)TiO2.yZrO2} wherein 3.5xe2x89xa6xc3x97xe2x89xa64 and 0xe2x89xa6yxe2x89xa60.2, a components added to the main component comprising about 5 to 30 parts by weight, on a ZnO basis, of a zinc compound, about 0.5 to 6 parts by weight, on a SiO2 basis, of a silicon compound, about 0.1 to 3 parts by weight, on an oxide (R2O) basis, of an alkali metal compound wherein R is alkali metal, about 0.1 to 7 parts by weight, on a CuO basis, of a copper compound, and about 0.1 to 6 parts by weight of, on a V2O5 basis, vanadium compound or on a Bi2O3 basis, a bismuth compound.
In this dielectric ceramic composition, the specific amounts of the auxiliary components are compounded to the major component represented by the above formula. This dielectric ceramic composition can be sintered at 1,000xc2x0 C. or less, and exhibits a high dielectric constant, that is, of approximately 30 or more, and a high Q value, that is, of approximately 2,000 or more at 8 GHz.
Since the dielectric ceramic composition does not contain B2O3, the composition exhibits high stability in high-temperature, high-humidity environments.
Since the dielectric ceramic composition can be sintered at lower temperatures, the composition can be simultaneously sintered together with a low-resistance inexpensive metals, such as silver and copper. Simultaneous sintering of the dielectric ceramic composition of the present invention and the low-resistance metals facilitates miniaturization of monolithic ceramic electronic components.
Reasons for the limitation of the composition in the present invention are as follows. At x less than 3.5 or x greater than 4.5, the temperature coefficient of the resonant frequency significantly shifts to the positive side, and the dielectric ceramic composition exhibits a large dependence of the dielectric constant on the temperature.
When ZrO2 is partially replaced with TiO2, the temperature coefficient of the resonant frequency shifts to the negative side. At y greater than 0.2, however, the dielectric constant xcex5 and the Q value decrease.
Preferably, 4.30xe2x89xa6xc3x97xe2x89xa64.4 and 0 less than yxe2x89xa60.1. In this case, the composition is composed of a Ba2Ti9O20 single phase, and exhibits a higher Q value.
The zinc compound contributes to an increased Q value and enhanced sinterability at low temperatures. When the content of the zinc compound is less than about 5 parts by weight, the composition does not have these advantages. When the content exceeds about 30 parts by weight, the dielectric constant xcex5 decreases. Preferably, the zinc compound is added in an amount of about 6 to 13 parts by weight.
When the content of the silicon compound is less than about 0.5 parts by weight, humidity resistance of the composition decreases. When the content exceeds about 6 parts by weight, the composition cannot be sintered at a temperature of 1,000xc2x0 C. or less. Preferably, the silicon compound is added in an amount of about 1 to 4 parts by weight.
The alkali metal compound is added in order to enhance sinterability at low temperatures. When the content thereof exceeds about 30 parts by weight, humidity resistance decreases. When the content is less than about 0.1 parts by weight, the composition cannot be sintered at low temperatures. Preferably, the alkali metal compound is added in an amount of about 0.5 to 2 parts by weight.
The vanadium or bismuth compound enhances sinterability at low temperatures. When the content thereof exceeds about 6 parts by weight, humidity resistance decreases. When the content is less than about 0.1 parts by weight, the composition cannot be sintered at low temperatures. Preferably, the vanadium compound is added in an amount of about 0.5 to 2 parts by weight, and the bismuth compound is added in an amount of about 0.5 to 3 parts by weight.
The copper compound contributes to enhanced sinterability at low temperatures and an increased dielectric constant xcex5. When the content thereof is less than about 0.1 part by weight, the composition does not exhibit these advantages. When the content exceeds about 7 parts by weight, the Q value decreases. Preferably, the copper compound is added in an amount of about 2 to 6 parts by weight.
The dielectric ceramic composition in accordance with the present invention preferably contains about 0.5 parts by weight or less and more preferably about 0.3 parts by weight, on a MnO2 basis, of a manganese compound and about 1.2 parts by weight or less and more preferably about 1.0 parts by weight, on a Ta2O5 basis, of a tantalum compound, as auxiliary compounds. The manganese compound and the tantalum compound contribute to a further increased Q value. The Q value, however, decreases when the content of the manganese compound exceeds about 0.5 parts by weight or when the content of the tantalum compound exceeds about 1.2 parts by weight.
At least one of the zinc compound, the silicon compound, the alkali metal compound, copper compound, and the bismuth compound is vitrified and added to the dielectric ceramic composition in accordance with the present invention. Since the reactivity to the major component is further enhanced, the composition can be sintered at even lower temperatures.
Any zinc compound, any silicon-compound, any alkali metal compound, any copper compound, any vanadium compound, any manganese compound and any tantalum compound may be used without limitation in the present invention.
Examples of zinc compounds include ZnO, ZnCl2, ZnS, Zn, Zn2SiO4 and Ba3Ti12Zn7O34.
Examples of silicon compounds include SiO2, Na4SiO4 and Si.
Examples of alkali metal compounds include Li2O, Li2SO4.H2O, Li3PO4, LiNO3, Li2C2O4 and Li2CO3. The alkali metals in the alkali metal compounds are not limited in the present invention, and include lithium, sodium and potassium. Among them, lithium is preferable.
Examples of copper compounds include CuO, Cu, CuSO4, Cu2O and CuCl.
Examples of vanadium compounds include V2O5, VOCl3 and VOCl2.
Examples of bismuth compounds include Bi2O3, BiCl3, Bi and BiC6H5O7.
Examples of zinc compounds include MnO2, MnCO3 and MnCl2.4H2O.
Examples of tantalum compounds include Ta2O5, TaCl5 and Ta.
As described above, the dielectric ceramic composition in accordance with the present invention comprises the major components and the auxiliary components. In the production of the dielectric ceramic composition, raw materials for the major and auxiliary components are weighed, are pulverized, compounded, calcined, and repulverized to form calcined powder. The calcined powder is molded into a predetermined shape and is sintered.
The dielectric ceramic composition in accordance with the present invention can be sintered at a low temperature, that is, at not more than 900xc2x0 C. in an air or under an oxygen atmosphere.
The raw materials may be hydroxides, carbonates or nitrates, which form oxides during sintering.
According to another aspect of the present invention, a monolithic ceramic substrate comprises a ceramic substrate including dielectric ceramic layers comprising the above dielectric ceramic composition and a plurality of internal electrodes formed in the dielectric ceramic layers. In this monolithic ceramic substrate, the dielectric ceramic layers are formed of the dielectric ceramic composition in accordance with the present invention and include the plurality of internal electrodes. Thus, the monolithic ceramic substrate can be sintered at a low temperature of 1,000xc2x0 C. or less, and exhibits a high dielectric constant, a high Q value and a small rate of change in dielectric characteristics with temperature.
Specifically, a second ceramic layer having a dielectric constant which is lower than that of the dielectric ceramic layers is formed on at least one face of each of the dielectric ceramic layers.
In a preferred embodiment of the present invention, the plurality of the internal electrodes are stacked with at least part of the dielectric ceramic layers provided therebetween to form a monolithic capacitor.
More specifically, the plurality of internal electrodes comprise internal capacitor electrodes constituting a capacitor deposited with at least part of the dielectric ceramic layers therebetween, and coil conductors constituting a laminated inductor by being connected to each other.
According to another aspect of the present invention, a ceramic electronic component comprises the above monolithic ceramic substrate and at least one electronic element mounted on the monolithic ceramic substrate and constituting a circuit together with the plurality of internal electrodes.
Preferably, a cap is fixed to the monolithic ceramic substrate so as to surround the electronic elements. More preferably, a conductive cap is used as the cap.
More specifically, the ceramic electronic component further comprises a plurality of external electrodes formed only on the bottom face of the monolithic ceramic substrate, and a plurality of through hole conductors electrically connected to the external electrodes and to the internal electrodes or the electronic element.
According to another aspect of the present invention, a monolithic ceramic electronic component comprises a sintered ceramic body comprising the above-described dielectric ceramic composition, a plurality of internal electrodes arranged in the sintered ceramic body and a plurality of external electrodes formed on outer surfaces of the sintered ceramic body, each being electrically connected to one of the internal electrodes.
Specifically, the plurality of internal electrodes are overlaid with the ceramic layers provided therebetween so as to form a capacitor unit.
More specifically, the plurality of internal electrodes comprises the internal electrode constituting the capacitor unit and a plurality of coil conductors which are electrically connected to form a laminated inductor unit.